Note: this is an early draft of the specification, do not take anything on this page for granted. Pretty much everything is open for discussion.

Introduction

This document specifies the OpenRaster Layer Stack XML format. The main two goals of the specification are interoperability between applications and allowing files to be saved without destructive pixel operations.

Note that this document does not define how the data is stored, and especially doesn't address the issue of how to store pixels nor how OpenRaster files can be stored on disk. This document doesn't address the mathematics behind layer compositing operations: it's simpler to describe that using other standard bodies' documents at this stage.

The first part of the document explains the concept of a baseline Layer Stack, with examples. Each attribute and element defined by the baseline profile is explained individually.

Baseline documents

A baseline OpenRaster document is a document that can be opened and look similar on all applications and platforms. The baseline layer stack format defines the minimum requirement to achieve interoperability and what is unsafe for interoperability. Layer Stack format permits extensions, which should be individually specified in detail, and work safely with the baseline profile.

Some users will require full interoperability, to create files which can be edited on every application supporting OpenRaster. Other users will only need to view OpenRaster files. This specification defines two classes of baseline support:

"Full baseline", which allows reading and editing

"Viewing baseline" which guarantees visualization of the file, but editing might damage the result

Syntax

The formal syntax of the layer stack is given following the RelaxNG schema:

Elements and Attributes

image element

This is the root element of the file. The logical size of the image is given by the mandatory "w" and "h" attributes, which are positive integers. The content expressed within image may have extents which can be smaller or larger, and so the image should be cropped to (0,0,w,h) when displaying, printing, or otherwise exporting to a context which requires a rectangular image.

The mandatory "version" attribute specifies the version of the OpenRaster specification to which the OpenRaster file as a whole conforms. Values are xsd:strings which conform to SemVer 2.0.0. (Since: 0.0.1)

The optional xres and yres attributes specify the nominal resolution of the document as a whole in the horizontal and vertical dimensions, in pixels per inch. Values are xsd:ints with a minimum value of 1. The default value, if unspecified, is 72. If either is specified, the other must also be specified. Applications should preserve resolution information specified in the document in both directions unless it is adjusted by the user, even if they make the simplifying assumption that xres is equal to yres. (Since: 0.0.3)

Common attributes for layer and non-root stack elements

x and y attributes

Position attributes. These are signed integers with a default value of 0.

name attribute

The name of the object represented by an element. A Unicode string, encoded in the XML document's own encoding.

opacity attribute

The object's opacity, expressed as a simple floating-point number. Values between 0.0 for fully transparent, and 1.0 for fully opaque.

visibility attribute

The visibility of the object.

Valid values are either visible, or hidden.

Default value: visible.

composite-op attribute

The operation to use when rendering this stack or layer over its backdrop. The effect is to be calculated as described in Compositing-1: General Formula for Compositing and Blending, namely as combinations of a colour blending function and a Porter-Duff alpha compositing operator. This attribute's name is perhaps a misnomer now, but it reflects the history of the OpenRaster specification and the evolution of the attribute from its inspirations in the earlier SVG 1.2 Working Draft and in GEGL's named processing "operations", which also provide the seeds of the naming scheme for values below.

The default value is svg:src-over, which represents simple alpha compositing.

In the future other compositing modes might be added, and a way for applications to define new modes will be specified.

stack element

The stack element describes a group of layers. They may contain sub-stacks, layers, or text elements. The first element in a stack is the uppermost.

The following attributes are optional on non-root stacks, but must be omitted on the root stack.

name

x and y

opacity

visibility

composite-op

layer element

The layer element defines a graphical layer within a layer stack, stored in a separate file within the OpenRaster file. The following attribute is required:

"src": the path to the stored data file for this layer. See the File Layout Specification for an explanation of the values which can go here.

The following attributes are optional on layer elements:

name

x and y

opacity

visibility

composite-op

text element

TODO: define it! Ideally, use another rich text specification, e.g. a relevant subset of the OpenDocument Text specification or XHTML.

Compositing the image

Layer stacks should be composited in a manner conforming to the W3C's Compositing and Blending Level 1 Candidate Recommendation. In terms of this specification's rendering model, some OpenRaster layer stacks or nested sub-stacks are isolated groups, but some sub-stacks may be non-isolated.

Isolated groups are always rendered independently at first, starting with a fully-transparent 'black' backdrop (rgba={0,0,0,0}). The results of this independent composite are then rendered on top of the group's own backdrop using the group's opacity and composite mode settings. Conversely non-isolated groups are rendered by rendering each child layer or sub-stack in turn to the group's backdrop, just as if there were no stacked group.

The root stack has a fixed, implicit rendering in OpenRaster: it is to composite as an isolated group over a background of the application's choice.

Non-root stacks should be rendered as isolated groups if: a) their isolation property is isolate (and not auto); or b) their opacity is less that 1.0; or c) they use a composite-op other than svg:src-over. This inferential behaviour is intended to provide backwards compatibility with apps which formerly didn't care about group isolation.

Applications may assume that all stacks are isolated groups if that is all they support. If they do so, they must declare when writing OpenRaster files that their layer groups are isolated (isolation='isolate'). (Since: 0.0.4)